1. Field of the Art
This invention relates generally to resin compositions and heat-sealable biaxially-stretched composite films having improved heat sealing and other characteristics.
More specifically, the invention relates to resin compositions of specific compositions each formed by blending an olefin-butene-1 copolymer of a specific composition and a crystalline polybutene-1 with a propylene copolymer of specific composition and relates also to biaxially-stretched polypropylene films onto at least one surface of each of which a resin composition of this character is laminated. Thanks to the low-temperature heat-sealing characteristics of the resin compositions, a composite biaxially-stretched polypropylene film is useful particularly for overwrap packaging.
Biaxially-stretched polypropylene films to which heat sealability has been imparted are endowed with the transparency, rigidity, strength, moisture-proof property, and other favorable properties possessed by biaxially-stretched polypropylene and are being widely used in the packaging field.
Among biaxially-stretched polypropylene films to which heat sealability has been imparted by various techniques, particularly those to whose both surfaces low-temperature heat sealability has been imparted are being used in the field of overwrap packaging. A film for overwrap packaging is used for the overwrapping of commodities which are principally packaged in paper boxes, such as tobacco and cigarettes, pharmaceutical products, soaps, and caramels.
Since overwrap packaging, in general, is carried out at a rate of 200 to 400 packages per minute by means of high-speed automatic packaging machines, a degree of suitability for automatic packaging is required of the wrapping material. More specifically, the prime requirement in automatic overwrap packaging is low-temperature heat sealability of the wrapping material. Since the weight of one packaging unit or piece of a commodity to be packaged as mentioned above, in general, is less than 100 grams (g.), the heat seal strength is not required to be very high, but it is necessary that a heat seal have a heat seal strength higher than 200 g./2 cm. of width even in the vicinity of 100.degree. C.
The next most important requirements in automatic overwrap packaging are that the wrapping film material possess rigidity, strength, good slip characteristic, and antistatic property so that the film will not bend or fold or break during its feeding since the film is fed at high speed into the packaging machine. Particularly when the slip characteristic and the antistatic property are deficient, automatic high-speed packaging with the film material in a packaging machine cannot be carried out. In addition to the above described suitability for automatic packaging, a film for overwrap packaging is required to have transparency for imparting an aesthetically pleasing appearance to the packaged commodity and, further, to have moistureproof property for protecting the commodity.
2. Prior Art
One of the known methods for imparting low-temperature heat sealability to a biaxially-stretched polypropylene film for the use of this film in overwrap packaging is the so-called coating method in which a biaxially-stretched polypropylene film which has once been processed and formed is coated with an agent for imparting heat sealability. For this agent for imparting heat sealability, solutions in organic solvents such as toluene, ethyl acetate, and methyl ethyl ketones of chlorinated polypropylene, polyvinylidene chloride, or the like as the predominant ingredient or coatings such as vinylidene chloride/acrylonitrile copolymer emulsions for application on an undercoat of an isocyanate adhesive solution can be used.
However, while the coating method can impart low-temperature heat sealability, the heat sealability imparting agent is applied as a coating in a process step separate from the process of producing the biaxially-stretched polypropylene film or a process step of drying off the organic solvent with hot air after the coating step. These additional steps not only add complications to the production process but also increase the production cost. Furthermore, since the heat sealability imparting agent is applied as a coating, not only is the transparency, which is an original advantageous feature of a biaxially stretched polypropylene film, impaired, but in the case where an organic solvent used, the organic solvent unavoidably remains in the coating layer, which is not a desirable state from the point of view of hygiene.
In order to avoid the above described high cost, residual solvent, and other problems of the coating method, there has been proposed a method wherein, in the production of a biaxially-stretched polypropylene film, a resin composition of a melting point lower than that of the base or substrate polypropylene is laminated to the polypropylene for imparting low-temperature heat sealability thereto, and then the laminated film is biaxially stretched. This lamination method is an excellent method since it has a higher productivity and simpler process steps than the coating method.
For the resin composition for imparting low-temperature heat sealability used in this method, ethylene resins and propylene resins have been proposed.
As ethylene resins, low-density polyethylenes and medium-density polyethylenes (as described in Japanese Patent Laid Open No. 101420/1974), ethylene-vinyl acetate copolymers (Japanese Patent Publication No. 11353/1966) and others have been proposed. In the case of an ethylene resin, however, while it is able to impart low-temperature heat sealability in correspondence with its low melting point, it is inferior to propylene resins in properties such as rigidity, transparency, hot slipping characteristic, and scratch resistance. Consequently, in the present state of the art, a composite, biaxially-stretched polypropylene film with a laminated layer of an ethylene resin which is amply satisfactory as a film for overwrap packaging has yet to appear.
On the other hand, propylene resins such as ethylene-propylene copolymers (as described in Japanese Patent Publication No. 31478/1971), propylene-butene-1-copolymers (as described in Japanese Patent Laid Open No. 128781/1975) and ethylene-propylene-butene-1 copolymers (as described in Japanese Patent Laid Open No. 35487/1974) have been proposed. In the case of propylene resins, however, while they are satisfactory on the points of rigidity, transparency, hot slipping characteristic, and scratch resistance, their low-temperature heat sealability is inferior because their melting points are higher in comparison with ethylene resins. Consequently, a composite, biaxially-stretched polypropylene film with a laminated layer of a propylene resin which is fully satisfactory as a film for overwrap packaging, also, has not yet been produced.
With the intent of improving the low-temperature heat sealability, which is a problem accompanying propylene resins to be used as agents for imparting heat sealability, compositions formed by blending propylene resins and polybutene-1 have been proposed as resin compositions for imparting low-temperature heat sealability. As one example, in Japanese Patent Laid Open No. 114482/1976, a method wherein by blending at least 5 percent by weight of a polybutene-1 with an olefin polymer and further adding a specific bisamide, the deterioration of transparency and the deterioration of heat sealability due to variation in the crystalline structure of the polybutene-1 are suppressed is described. As another example, Japanese Patent Laid Open No. 150560/1976 discloses an agent for imparting low-temperature heat sealability which is formed by blending 35 to 65 percent by weight of a polybutene-1 with a propylene-butene-1 random copolymer of a butene-1 content of 10 to 15 percent by weight. As still another example, Japanese Patent Laid Open No. 150561/1976 proposed an agent for imparting low-temperature heat sealability which is formed by blending 45 to 65 percent by weight of a polybutene-1 with an ethylene-propylene random copolymer of an ethylene content of 0.5 to 4.5 percent by weight.
However, to the best of my knowledge, it is difficult to say that these proposals are fully satisfactory. More specifically, in the method of the above cited Japanese Patent Laid Open No. 114482/1976, a specific bisamide must be added. In the case of the Japanese Patent Laid Open Nos. 150560/1976 and 150561/1976, the imparting of low-temperature heat sealability becomes possible only by blending a large quantity of at least 35 percent by weight of the polybutene-1, but the blending of such a large quantity of the polybutene-1 causes an impairment of the transparency and luster, which are characteristics originally possessed by a biaxially-oriented polypropylene film.
As indicated above, in the present stage of the art, a resin composition for imparting low-temperature heat sealability and a biaxially-stretched polypropylene film onto which this composition has been laminated as a layer, which are fully satisfactory for overwrap packaging, and in which the various desirable characteristics originally possessed by biaxially-stretched polypropylene films are not impaired, have not yet been produced.
In view of this state of the art, I have previously proposed the utilization of blended compositions of olefinbutene-1 copolymers of specific compositions and polybutene-1 as resins of low-temperature heat sealability (as disclosed in the specification of Japanese Patent Application No. 93907/1977).